This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
This is properly the sesquichloride or perchloride of iron. Though long and very largely used in alcoholic solution, and more recently, to a considerable extent, dissolved in water, this chalybeate has until of late never been kept in the dry state, and was for the first time introduced into the U. S. Pharmacopoeia at the last revision. It is prepared by first treating iron wire with muriatic acid, by which the protochloride is produced in solution; then bringing this to the state of sesquichloride by adding nitric acid, and a further quantity of muriatic acid, to the heated solution; and lastly evaporating and crystallizing The simplest explanation of the process is that, in the first step, iron attracts sufficient chlorine from the muriatic acid to convert it into the protochloride, while the hydrogen of the acid escapes with effervescence; and, in the second, the nitric acid gives up a portion of its oxygen to the hydrogen of the muriatic acid in excess, thereby liberating enough chlorine to bring the protochloride of iron to the state of the sesquichloride.
Thus procured, the chloride of iron is in crystalline fragments, of an orange-yellow colour, inodorous, of a strongly styptic and ferruginous taste, very soluble in water, deliquescent, and soluble in alcohol and in ether. It consists of two eqs. of iron and three of chlorine, and is represented by the formula FeaCls, with a variable quantity of water of crystallization. That it is a sesquichloride of iron, containing no protochloride, is proved by its affording a blue precipitate with the ferrocyanide, but none with the ferridcyanide of potassium. The only objection to it in the solid state is its liability to deliquescence, which can be guarded against by keeping it in accurately closed bottles. It undergoes no change on exposure. There are two officinal preparations of it; the tincture, which is chiefly employed internally; and the watery solution, which is used locally. It has been employed as a local styptic in a semi-deliquesced state, and found to be extremely efficient. For this purpose, Mr. J. Z. Lawrence keeps it in a bottle loosely closed, and, as it deliquesces, applies the thick liquid portion, by means of a spun-glass brush, to bleeding surfaces. MM. Jodin and Aubrun, of Paris, have used it internally in pseudomembranous croup, with great asserted success, giving it in quantities varying from ninety grains to half an ounce, in divided doses, very frequently repeated, in the twenty-four hours. (Ann. de Therap., 1861, p. 201.) But these I should consider as very hazardous doses, and justifiable only under desperate circumstances. The solid sesquichloride has proved an efficient remedy in that obstinate and painful ulceration about the toe-nail, resulting from the pressure of the edge of the nail upon the flesh. The powder is introduced beneath the protruding flesh, and sprinkled on its surface. The pain quickly subsides, and the patient is able to walk in a few days. (Bost. Med. and Surg. Journ., Oct. 22, 1863, p. 240).